Straight through silencer



Sept. l5, 1959 R. B. BOURNE ETAL STRAIGHT THROUGH sILENcER 2 Sheets-Sheet 1 Filed May 8, 1955 Ffa. l

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' STRAIGHT THROUGH SILENCER @WWW STRAIGHT THROUGH SILENCER YRoland B. Bourne, West Hartford, and John P. Tyskewicz, Hartford, Conn., assignors, by mesne assignments, to Emhart Manufacturing Company, Hartford, Conn., a corporation of'Delaware y Application May 8, 1953, Serial No. 353,803

6 Claims. (Cl. 181-42) This invention relates to improvements in silencers of the straight through type and, more specifically, to a silencer which is particularly adapted for application to the exhaust of a jet engine for test operations within a cell or for test operations of the engine after installation in an air frame. By straight through silencers is meant the type shown and described, for example, in U.S. Patent No. Re. 22,283. As shown in the patent, this type of straight through silencer comprises an open or unobstructed main sound and gas conducting channel having one or more laterally disposed acoustic side branches which are acoustically coupled to the main channel through a body of sound absorbing material. More specifically, this type silencer comprises a perforate main channel extending between end headers, a layer of sound absorbing material disposed contiguously around the perforate tube forming the main channel and a closed cavity surrounding the sound absorbing material and acoustically coupled to the main channel through the said sound absorbing material'.

The frequency-attenuation characteristics of this type of silencer resulted in more or less uniform sound attenuation over a very wide range of frequencies. It is characteristic of this type of silencer, as pointed out in said patent Re. 22,283, that a reduction in density of the sound absorbing material results in greater attenuation for some sound frequencies at the expense of reduced attenuation for other frequencies, the reduction being most noticeable in the low frequency region. The type of sound absorbing material utilized in this type of straight through silencer has a lot to do with performance, and it has been found that glass wool and generally similar materials have better acoustic properties than metallic Wool and the like.

In the adaptation of straight through silencers for jet engine purposes, it has been found impractical to use sound absorbing material such as glass Wool, etc. due to the very high sound levels occurring and due to the high temperatures encountered in the exhaust streams which Aoften require the use of copious quantities of cooling water. More specifically, the glass wool layers become clogged with the cooling water and have generally caused poor performance. Metallic wool, being heat resistant and less dense, has been found to be the best practical sound absorbing material for this particular type of installation.

Silencers adapted for jet engine use are generally quite large, the main channel being of the order of to 6 feet in diameter, and the use of metallic wool sound absorbing material, which is less dense than glass wool, etc.,

For example, the overall size of the silencer would become -l too large and a silencer constructed in accordance with Patented Sept. 15, 1959 ice s vsaid Reissue Patent 22,283 would have to be fabricated on location and could not be conveniently shipped by railwayor the like. Another solution of the poor performance characteristics might be accomplished by increasing the thickness of the metallic sound absorbing layer, but costs would in most cases be prohibitive.

Therefore, it is the general object of the presentinvention to provide a high-performance, economical silencer of the straight through type which is particularly adapted for use with jet engines or the like where the exhaust streams are at extremely high temperatures and where a very wide range of sound frequencies are encountered.`

, We have found that the desirable attenuation vs. frequency characteristics of the straight through silencers of the typeshown in said re-issue patent are preserved in a silencer construction utilizing metallic Wool as the sound absorbing material by closing off a portion of the `layer of sound absorbing material contiguous with the closed cavity, whereby the acoustic conductivity through which the cavity is coupled to the main channel is reduced, thus lowering the low frequency cut-off of the silencer, while preserving an acoustically lined duct for the attenuation of the high sound frequencies.

We have also found that the low frequency response is so improved by this means that the use of special low frequency traps of the volumetric low-pass filter type is largely obviated. Thus, by avoiding non-dissipative lters there isv considerably less likelihood of creating noises within the silencer. Certainly, there can be no sounding-off of the resonant chambers of a lter in the silencer of the present invention. By largelyeliminating this type of created noise within the silencer, we have increased the net noise reduction of the silencer.` Of

course, self-madeV noise induced by turbulent vortices along the surface of the inner perforate tube are unaffected by our construction.

Other vobjects and advantages of the invention will be apparent to those skilled in the art from the following description of the annexed drawings which are illustrative of several embodiments of the invention and in which,

Fig. l is a diagrammatic longitudinal cross-sectional view of the silencer constructed in accordance with U.S. Reissue Patent 22,283;

Fig. '2 is a similar diagrammatic view of a straight through silencer incorporating the features of the present invention;

Fig. 3 is a similar diagrammatic view of an alternative silencer constructed in accordance with the present invention;

Fig. 4 is a similar diagrammatic view of another silencer embodiment constructed in accordance with the present invention; t

Fig. 5 is aA similar diagrammatic view of still anothe embodiment;

Fig. 6 is a diagrammatic transverse cross-sectional view of the silencer shown in Fig. 5 taken as indicated by the line 6-6;

Fig. 7 is a diagrammatic llongitudinal cross-sectional -view of a silencer incorporating all of the specific features of the embodiments shown in Figs. 2 to 6; and Figs. 8 -to l0 show graphs illustrating the performance of thev various silencer embodiments.

As shown in Fig. 1, the conventional straight through silencer of the type described -in U.S. Patent Re. 22,283 comprises a generally cylindrical shell 10 tted within 'end headers 1,1 and 12 and having a main sound and gas conducting channel 14 formed by the perforate tube 16 extending between the heads 11 and I12. An outer vperforate tube 18 surrounds the tube 16 -in spaced concentric relationship and a layer of sound absorbing material 20 is disposed between said perforate tubes 15 and 18. The closed side branch chamber 22 is formed between the cylindrical shell and the outer perforate tube 18 and between the end headers 11 and 12, said Vside branch chamber being :acoustically coupled with the main channel 14 through the =layer `of soundabsorbing material 2t?.

Curve A of Fig. 8 shows a plot of attenuation vs. frequency bands for the silencing device shown in Fig. 1 wherein metallic wool was used as the sound absorbing material and wherein the diameter and length of the cylindrrical shell ltl are each equal to three times the diameter o-f the main channel 14. The silencer used in plotting the curve A had a main channel diameter of 6 inches and the measurements were made with a sound source comprising a random noise generator and suitable ampliers and loud speakers feeding `into a iinite acoustic conduit. Microphones were positioned in the acoustic channel upstream and downstream of the silencing device.

In extrapolating the curve to large size silencers a linear proportioning scale is used. For example, in extrapolating to a 60 inch diameter conduit the octave band frequencies are divided by l0. This is permissible for all frequencies up to the relatively high frequencies where the projection effect of beaming the sound waves through the main channel takes place, taking care, at the same time, to likewise scale the acoustic properties of the dissipative acoustic material.

In -accordance with the present invention, the silencer embodiment shown in Fig. 2 is particularly adapted to preserve the desirable features of conventional straight through silencers as described in Re. 22,283 when metallic wool is used as the sound absorbing material. More specifically, the silencer of Fig. 2 comprises a cylindrical shell 24 closed by end -headers 25 and 26 which receive a centrally located longitudinally extending perforate tube 27 which defines the main sound and gas conducting channel 28. The shell, headers and inner perforate tube are similar in all respects to their counterparts shown in Fig. l.

A layer of metallic wool sound absorbing material 29 surrounds the tube 27. An outer parti-perforate tube 30 surrounds the sound absorbing pack 29. While the tube 3d is equal in size to the previously described outer perforate tube 13, thereby limiting the thickness of the sound absorbing pack layer 29 to the thickness of the previously described layer 20, the said tube 30 comprises two imperforate sections 31, 31 which are separated by a. cylindrical perforate section 32. In all embodiments shown herein, the type of perforation and percentage of open area in both the inner land outer tubes is substantially the same. That is, the perforate portion of the outer tube and the perforate inner tube are substantially alike as to the perforations. Prefer-ably, the perforate areas described are 40% open. As `shown in the drawings, the perforate section 32 constitutes approximately one-third of the surface area of the outer tube 35) and the imperfor-ate sections 31, 31 constitute the remaining two-thirds. ln other words, acoustic coupling between the main channel Z3 and the coustic side branch chamber 33, defined between the tube 30 and the shell 24, is provided through only a portion of the surface area of the outer perforate tube 3G. In the illustration, the said acoustic coupling takes place through approximately onethird of the surface area of the outer tube. Satisfactory results will occur when approximately 25% to approximately 50% of the total surface area of the outer tube is exposed for acoustic coupling.

The 'attenuation vs. frequency performance of the silencer of Fig. 2, which is equal in size to the first silencer tested, is plotted by curve B of Fig. 8 for comparison with the performance of the conventional straight through silencer of Fig. l which used metallic wool as the sound absorbing material. It will be noted that curve B indicates a decided improvement in the sound attenuation al performance of the silencer of Fig. 2 as compared to the curve A plot of the silencer of Fig. l. The improved performance characteristic is most noticeable `in the low frequency end of the sound spectrum.

Fig. 3 shows a silencer of the same dimensions as the silencer of Figs. l and 2. Accordingly, the same reference characters are used to denote elements in Fig. 3 which are identical with the elements shown in Fig. 2. As will be noted in Fig. 3, a modified parti-perforate tube 3S is `used in place of the previously `described parti-perforate .tube 30. The tube 35 includes a single imperforate section 35 which comprises approximately two-thirds of the total length of the tube and a perforate section 37 comprising approximately one-third of the tube at one end thereof.

The attenuation v. lfrequency performance of the embodiment shown in Fig. 3 is plotted by curve C of Fig. 8. It will be noted that the attenuation peak in the low frequency spectrum occurs in the -300 octave band showing that the action of the side branch is of the natu-re of a linear damped resonator.

Fig. 4 shows a silencer section approximately one-half the length of the previously described silencers but having equal diametrical measurements. Said silencer section is formed by a cylindrical shell 40 closed at both ends by headers 41-42 which receive a centrally located longitudinally extending perforate tube 43 which defines the main gas and sound conducting channel 44. An outer perforate tube 4S surrounds the inner perforate tube 43 in spaced relationship and a metallic wool sound absorbing pack 46 is disposed between the said perforate tubes. The cavity within the shell 49 around the outer perforate tube 45 is divided into three chambers, namely, a main annular chamber 47 and a pair of smaller annular chambers 4d, 48 defined by the frusto-conically shaped annular members Si), 50 which extend between the outer perforate tube 4S and the headers 41 and 42. In this embodiment, the frusto-conically shaped members Si), 50 close olf communication between the main side branch chamber 47 and the inner perforate tube so as to reduce the acoustic coupling therebetween. In addition, due to the small volume and shape of the chambers 48, 48, attenuation is achieved of a group of sound frequencies much higher than those attenuated by the main side brunch chamber 47. This performance characteristic is plotted by curve E in Fig. 9. For purposes of comparison, the silencer of Fig. 4 was tested with the members Sti, Si? removed resulting in the performance plotted by curve D in Fig. 9.

Another alternative construction embodying the principles of the present invention is shown in Figs. 5 and 6 wherein the cylindrical shell, the headers and the inner perforate tube are identical with their counterparts shown in Fig. 4 and are denoted by the same reference characters. However, in this embodiment a modified outer tube S1 is used in place of the previously described outer perforate tube 4S. The tube 51 comprises a longitudinally extending perforate section 52 and a longitudinally extending imperforate section S3. The perforate section extends through the total length of the tube 51 but encompasses approximately one-third of the surface area thereof. While the acoustic coupling area between the main channel 44 and the side branch chamber 54, defined between the tube 51 and the shell 40, is the same for the device shown in Figs. 5 and 6 as for the device shown in Fig. 4, the performance differs somewhat as can be seen from curve F, Fig. 9.

Naturally, the attenuation of a single silencer or single silencer section of given length is generally insufficient to meet the requirements of installations wherein a very -wide range of sound frequencies is encountered. Therefore, it is expedient to combine several sections of the same or alternative constructions of the type shown in series relationship to attain the desired attenuation. The embodiments shown in Figs. 2, 3 and 5 lend themselves particularly well to series combinations because, of the total longitudinal extent of the individual sections, only a portion of each section is acoustically coupled to the main channel, the remainder of each section serving as a dissipative coupling channel between sections to the end that series resonance is reduced.

The effect of series combination Iwas pointed out in the U.S. patent to Bourne, 2,051,515. However, the devices of said patent are not to be confused in consideration of the present invention, because, in accordance with the present invention, the acoustic coupling to the cavities outside the main channel is accomplished through a body of sound absorbing material which also forms an acoustic part of the main channel. Due to the presence of the sound absorbing material in the coupling zone, the frequency response ofthe side chamber is broadened as compared with the patented devices and a greater area is provided in the coupling Zone with a consequent better grip on the sound frequencies in the main channel. ln addition, there is provided in accordance with the present invention considerable protection against shell noise in the outer casing. While it may be said that protection against shell noise is afforded in accordance with the teachings of Reissue Patent 22,283, it is even more manifest in the devices or" the present invention due to the limited and dissipative nature of the coupling area between the main channel and the outer casing.

Fig. 7 shows a composite structure embodying the four different section types previously described. As shown, there are seven sections indicated by the reference characters 100 to 106 surrounding a single perforate tube 110 which defines a main gas and sound conducting channel 120. The sections 100 and 103 are similar to the embodiment shown in Fig. 3 but differ in length therefrom and from each other. Section 101 is similar to the embodiment shown in Fig. 2 but of different length and the section 102 differs only in length from the embodiment shown in Fig. 5. Sections 104, 105 and 106 are similar to the embodiment shown in Fig. 4.

A typical attenuation V. frequency plot for a composite silencer is shown in Fig. 10 wherein the sound frequencies plotted are of the order encountered in large silencers having a main channel diameter of the order of five to six feet. -It is a silencer of this type and size which may be advantageously applied to the exhaust of a jet engine or the like. Noise reduction is plotted against sound frequency for the purpose of more graphically illustrating performance. Noise reduction as used in Fig. 10 takes into account the self-generated noise caused by the passage of high velocity gas through the silencers from the jet engine exhaust and also takes into account the attenuation of the silencer. IIn addition, Fig. 10 takes into account the short-circuiting of sound between sections which takes plaec through the transverse headers separating the sections as well as through the shell of the silencer.

While other modications may be effected within the scope of the present invention, our findings indicate that best results will be obtained in all modified forms if the coupling area between the main channel and the side branch chamber or chambers falls within the range of 25% to 50% of the surface area of said main channel.

Since it is believed that other alternative constructions may fall within the scope of the invention, it is not intended to limit the invention to the specific embodi ments shown and described otherwise than indicated by the claims which follow.

We claim as our invention:

1. A sound attenuating device comprising an imperorate cylindrical shell, -a pair of imperforate headers closing the ends of the shell each of which headers has a centrally disposed gas flow opening, an unobstructed perforate tube extending from the opening in one header to the opening in the other to provide a main sound and gas conducting channel through the device, a layer of sound absorbing material extending between the headers and completely surrounding the said channel-defining tube, and an outer tube surrounding the sound absorbing material and contiguous therewith and disposed between the said headers in spaced relationship to the shell, the said outer tube being imperforate except for a perforate portion comprising approximately one-third of its surface area in which perforate portion the surface area is substantially 40% open to provide acoustic communication through the sound 'absorbing material between the main sound and gas conducting channel and the sidebranch space defined between said outer tube and the shell.

2. A sound attenuating device as deiined in claim 1 wherein the perforate section of said outer tube comprises a cylindrical portion of the tube disposed substantially midway between said headers.

3. A sound attenuating device as defined in claim 1 wherein the perforate section of said outer tube comprises a cylindrical portion thereof disposed adjacent one of said headers.

4. A sound attenuating device comprising an imperforate cylindrical shell, a pair of imperforate headers closing the ends of the shell each of which headers has a centrally disposed gas flow opening, an unobstructed perforate tube extending from the opening in one header to the opening in the other to provide a main sound and gas conducting channel through the device, a layer of sound absorbing metallic wool extending between the headers and completely surrounding the channel-defining tube, and `an outer tube surrounding the layer of wool and contiguous therewith and disposed between the headers in spaced relationship to the shell, the said outer tube being imperforate except for a perforate portion comprising one-third of its surface area in which perforate portion the surface area is substantially 40% open to provide acoustic communication through the sound absorbing material between the main sound and gas conducting channel and the sidebranch space defined between said outer tube and the shell.

5. A sound attenuating device as defined in claim 4 wherein the perforate section of said outer tube comprises a longitudinally extending portion thereof extending between said end headers.

6. A sound attenuating device comprising an imperforate generally cylindrical shell, a pair of imperforatc headers closing the ends of the shell each of which headers has a gas flow opening, an unobstructed perforate tube extending from the opening in one end header to the opening in the other to provide a main sound and gas conducting channel through the device, a layer of sound absorbing material completely surrounding the said tube and generally cylindrical means surrounding the said sound absorbing material and contiguous therewith and disposed between the end headers in spaced relationship to the shell, the said means being imperforate except for a perforate portion comprising from 25% to 50% of its surface area in which perforate portion the surface area is substantially 40% open to provide acoustic communition through the sound absorbing material between the tube and the sidebranch space between said means and the shell.

References Cited in the le of this patent UNITED STATES PATENTS 681,522 Very Aug. 27, 1901 722,567 Crawford Mar. 10, 1903 927,246 Jackson July 6, 1909 2,043,731 Bourne June 9, 1936 2,051,515 Bourne Aug. 18, 1936 2,056,608 Jack Oct. 6, 1936 2,148,948 Kingsley Feb. 28, 1939 2,176,615 Starkweather et al Oct. 17, 1939 FOREIGN PATENTS 881,494 France Jan. 28, 1943 

